1. Technical Field
The present invention relates to a liquid crystal display device (LCD), and more particularly, to an LCD that mitigates electromagnetic interference (EMI), and a fabrication method thereof.
2. Related Art
In general, display devices have used a cathode ray tube (CRT) to display image information on a screen. However, CRT displays are problematic as they take up a large volume and are extremely heavy compared with the display area.
With the rapid development of the electronics industry, the importance of the CRT display devices has decreased. Various electronic devices such as a personal computer, a notebook, a wireless terminal, a vehicle instrument panel, an electronic display board and the like now require much lighter and smaller displays than that afforded by the CRT. Also, due to the development of information communication technology, since it is possible to transmit large capacity image information, the importance on a next generation display device capable of processing and displaying the large capacity image information has increased dramatically.
Such a next generation display device ideally has a slim profile, is lightweight, has a high luminance, a large-sized screen, a low power consumption and a low price. Among such next generation display devices, the liquid crystal display (LCD) is extremely popular due to its excellent resolution, which is better than other flat displays, as well as the fact that its response rate is fast, that is, considerable to that of the CRT in implementing a moving picture.
The LCD generally displays images using light transmittance that varies when a voltage is applied to electrodes of two substrates facing each other to generate an electric field between the electrodes. This electric field aligns liquid crystal molecules disposed between the electrodes.
The LCDs can be made in a variety of structures and materials. An active matrix LCD (AM-LCD) including thin film transistors (TFTs) and pixel electrodes connected with the TFTs is currently in the limelight owing to its excellent resolution and moving picture-implementing capability.
FIG. 1 is a sectional view of a general LCD. Referring to FIG. 1, a general LCD includes an array substrate 110, a color filter substrate 120, and a liquid crystal layer 130 interposed between the two substrates 110 and 120.
The array substrate 110 includes a gate electrode 112 and a gate insulating layer 113. The gate electrode 112 is formed of conductive material such as metal on a first transparent substrate 111. The gate insulating layer 113 covers the gate electrode 112 and is formed of silicon nitride layer (SiNx) or silicon oxide layer (SiO2).
An active layer 114 is formed on the gate insulating layer 113, and an ohmic contact layer 115 is formed on the active layer 114. The active layer 114 is formed of undoped (i.e. unintentionally doped) amorphous silicon, and the ohmic contact layer 115 is formed of intentionally doped amorphous silicon.
Source and drain electrodes 116a and 116b are formed on the ohmic contact layer 115 and constitute a thin film transistor T together with the gate electrode 112. The source and drain electrodes 116a and 116b are formed of conductive material such as metal.
Although not shown, the gate electrode 112 is connected to a gate line, and the source electrode 116a is connected to a data line. The gate line and the data line are perpendicular to each other. The gate line and the data line define a pixel region.
A passivation layer 117 is formed on the source and drain electrodes 116a and 116b and it has a contact hole 117c exposing the drain electrode 116b. The passivation layer 117 is formed of a silicon nitride layer, a silicon oxide layer, or an organic insulating layer.
A pixel electrode 118 is formed on the pixel region and is connected to the drain electrode 116b through the contact hole 117c. The pixel electrode 118 is formed of a transparent conductive material.
The color filter substrate 120 is spaced apart from the first substrate 111 by a predetermined distance. The color filter substrate includes a second substrate 121. A black matrix layer 122 is formed in an inner side of the second substrate 121 at a position corresponding to the thin film transistor T. Although not shown, the black matrix 122 is formed on a front side of the substrate and has an opening at a position corresponding to the pixel electrode 118.
Accordingly, the black matrix layer 122 prevents light leakage, which occurs when liquid molecules are tilted in a region other than the region covered by the pixel electrode 118. Also, the black matrix layer 122 blocks light from being incident on a channel region of the thin film transistor T, thereby preventing light leakage current.
A color filter 123 is formed on the black matrix layer 122. The color filter layer 123 includes red, green and blue color filters, which are repeated in sequence. One color corresponds to one pixel electrode 118. A common electrode 124 is formed of a transparent conductive material below the color filter layer 123.
A method for manufacturing the LCD includes forming a thin film transistor and a pixel electrode on an array substrate, forming a color filter layer and a common electrode on a color filter, substrate, arranging the substrates, injecting liquid crystal between the substrates, sealing the substrates, and attaching polarizers to the substrates. In such an LCD, the pixel electrode 118 is formed on the array substrate 110 and the common electrode is formed on the color filter substrate 120. Liquid crystal molecules are driven by an electric field applied in a direction perpendicular to the substrates.
FIG. 2 is a view schematically illustrating a portion of a color filter substrate in an LCD according to the related art. Only one edge of the second substrate is shown in FIG. 2. In FIG. 2, a color filter substrate includes an active region A, which corresponds to an image display region formed on the array substrate. Electrical signals are applied to a variety of elements using a region formed at a periphery of the active region.
As shown, a black matrix layer 122 is formed on the color filter substrate. The black matrix layer 122 prevents light leakage between cells of color filter patterns and at a periphery of the active region A.
Patterns of red (R), green (G) and blue (B) color filters 123 are formed on the active region A of the black matrix layer 122. An overcoat layer 130 for insulation and planarization of the layers thereunder is coated to a predetermined thickness on the substrate on which the color filter patterns are formed. A common electrode 124 is formed on the overcoat layer 130 so as to operate liquid crystal cells together with a pixel electrode formed on a lower TFT substrate.
In order to apply signals from a pad part of a lower substrate to the common electrode 124, a common voltage (Vcom) Ag dot 142 is formed at a position that is in contact with the common electrode 124 formed on the black matrix 122.
FIG. 3 is a partial sectional view of the color filter substrate, taken along line I-I′ of FIG. 2. Referring to FIG. 3, the color filter substrate includes the black matrix layer 122 formed on the transparent insulating substrate 121, the color filter layer 123 of red, green and blue color filters, the overcoat layer 130, and the common electrode 224, which are sequentially formed on the black matrix layer 122. As shown, the Vcom Ag dot 142 is formed at a position that is in contact with the common electrode 124 formed on the black matrix 122.
A conventional LCD with such an arrangement has a good transmittance and aperture ratio. The common electrode 124 acts as ground and prevents breakdown in the liquid crystal cells due to a charge buildup between the substrates. In the conventional LCD, however, a very fast electric change occurs when an image display signal is applied, so that a large amount of electromagnetic interference (EMI) occurs.
Although electromagnetic radiation is useful in wireless communication or radar applications, unintended electromagnetic radiation occurring inside a variety of electronic devices containing the LCD may influence the operation of the electronic devices containing the LCD as well as the operation of other electronic devices present. As a result, operation of the electronic devices may be interrupted or the electronic devices may otherwise malfunction. Further, if the electromagnetic components are radiated or transported through connections between the LCD and other parts of the electronic device, the picture or tone quality of the LCD may be degraded.